Thanks Monks, I think I got it. An HTTP POST sends data to a program on the other end (POST and EXECUTE). In my case I had PHP take the hand off. So in PHP-land the HTTP POST shows up as an Array of values delivered in $_POST. The PHP program takes it from there (array of passed values) and if coded it can write to a file or anything php can do. My Perl send off below:

my $req = new HTTP::Request 'POST','http://foobar.com/stuff/post.php';
$req->content_type('application/x-www-form-urlencoded');

my $text = "rambling text message here for future use, Dave can hear me. ";
my $user = "yourusernamehere";
my $pass = "yourpasswordhere";
my $number = "44.55";

$req->content("user=" . $user . "&pass=" . $pass . "&text=" . $text . "&number=" . $number);
my $res = $ua->request($req);
print $res->as_string;


So basically I had two misconceptions:
1) that POST could just write directly to a remote .txt file.
2) something else that escaped me at first is that echo and print statements at the server within the remote (php) program are returned to sender. I had the view that POST was sort of one way comms. But it's more POST-EXECUTE-REPLY.